Sorry, what was your question? Your tagline image is so large I can't see it.

But seriously, I think it depends on what you are looking to do (PvE or PvP). Xu-Fu has alot of possibilities in the PvP realm, or so I have heard (I've still done almost none). I haven't really tested out Xu-Fu or Chi-Chi against the tamers or Beasts of Fable, though.

Chi-Chi would be great against mechanicals (Wild Magic + Fire Quills), unfortunately there are not many of those in the tamer circuit (unless you like to go to Outlands for 19 gold in Hellfire Peninsula or the like).

Xu-Fu for levelling is not really as good as Gahz'rooki because his Prowl/Feeding Claws has too long a CD. Zao is in a similar boat - huge heal but the delay is too long for using it (perhaps) to be useful. They can both hit hard but might get hit too much to be as sustainable as Gahz'rooki (or the Molten Spider I use as my backup).

Yu'la would probably be excellent for levelling pets against a flying team. She's not as good as an emerald proto-whelp for the tamer circuit as her anti-flyer attack is an AoE.

Xu-fu is excellent for pet leveling against wild pets, provided you aren't fighting too many flyers. Attack with the low level pet, swap in Xu-fu, and just go to town. He starts getting low on life, just super-charge his attack with Prowl and then hit with the Feed. It heals him for damage done, so Prowl ups the healing from 350-400 to 800-900 (still not sure how that math works, but that's what I was seeing yesterday), leaving you with a fully healed team for the most part unless it misses. I spent 2 hours leveling pets and tracking my missing wild rares yesterday, only had to heal the pets twice, and that was for the Terrible Turnip, my third pet I brought in when finding a rare.

I think Xu-Fu is the easiest to get good use of, he is pretty powerful for wild pet battles, or just killing critters. All of them have some strong moves, but i have a hard time seeing where i would replace any of them with something i currently use.

Yu'la, Broodling of Yu'lon is outstanding vs any legendary pet. If you already have a strong stable of pets, I would argue it is the best choice in terms of usefulness. Xu-Fu is an exceptionally strong beast though, and I imagine Chi-Chi has his uses. Zao is probably the least desirable.

In this last week's tournament for example, I used the Yu'la -> Kneebiter combo vs Zao (1st turn blessing, 2nd turn life exchange, breath spam, black claw, hunting party) and overkilled him by an even more ridiculous amount than I had with the standard water spirit + chrominius howl combo previously, with absolutely no risk of losing.

Zao is probably the most unique of the four, since it's a beast with Wish. Chi-Chi isn't really all that special and Xu-Fu is just another DPS beast, while Yu'la doesn't really have any unique moves (Harmonious Porcupette also comes with Celestial Blessing and there are plenty of pets with Life Exchange).

While a harmonious porcupette also has blessing, the remainder of its moveset is sorely lacking vs most legendaries. By the same token, none of the life exchange pets have other compelling abilities. It's the combination of the two, plus the dragonkin passive (which can proc off your life exchange cast if you remain below 50% after the exchange heals you, by the way) that makes yu'la stand out. Yu'la's moveset is far more type-agnostic than the porcupette's as well.

Speed/health are also considerations, but usefulness due to stats between yu'la and harmonious is going to be very situational.